Many optical devices and sensors operate by sending a beam of light at a known wavelength, and the information collected by these devices and sensors depends on the wavelength being precisely known or, for some devices, stable in time. The wavelength-selective optical devices undergo mechanical drifts due to thermal gradients and aging, and thus a periodic wavelength referencing or calibration is required. By way of example, laboratory-grade optical spectrum analyzers (OSA) require periodic wavelength calibration to maintain the absolute wavelength accuracy stated in the OSA operation manual.
A wavelength calibration typically involves using an optical element having a known spectral property, such as an absorption gas cell having known absorption peaks, or a reference light source emitting light of a known wavelength. The reference light sources must provide light at a stable, or at least precisely known, wavelength(s). They have evolved from sources traditionally used in spectroscopy, such as inductively-coupled alkali-metal plasma cells, to more recently used reference laser sources.
A variety of reference laser sources are known. By way of example, Helium-Neon lasers emit light at a wavelength of 632.8 nm. One can also use a fiber-coupled laser diode having an external fiber Bragg grating as an optical feedback element to provide a reference laser source at wavelengths ranging from visible to near-infrared. To increase the wavelength accuracy of a semiconductor reference laser source, some form of thermal stabilization is usually required.
The simplest approach is to stabilize the temperature of the laser diode itself. This is achieved by placing the laser diode on a heat sink coupled to a thermoelectric cooler (TEC) element, measuring the temperature of the laser diode, and providing a feedback to the TEC element to stabilize the temperature of the laser diode. The wavelength stability of this approach is limited by a fluctuation of a temperature difference between the point of measurement of the temperature, which is usually located somewhere on the laser's packaging, and the semiconductor chip of the laser diode. The temperature difference depends on such parameters as thermal resistance between the semiconductor chip and the laser's package, and amount of heat supplied to the laser diode by the driving current.
A thermistor or a thermocouple are typically used to measure the laser diode temperature. It is also noted that Jay et al. in US Patent Application Publication 2004/0052299, and Shih in U.S. Pat. No. 7,052,180 have shown that a voltage drop across a semiconductor junction of a photodiode and a light-emitting diode, respectively, can be used to measure temperature of the semiconductor junction.
An incumbent approach to increasing wavelength stability and precision of a temperature-controlled laser diode has been to couple the laser diode to a so-called wavelength locker, for example to a temperature-stabilized Fabry-Perot etalon. When a lasing wavelength of the laser diode deviates from a set wavelength, the wavelength locker provides a feedback signal to a control circuit that changes the temperature of the laser diode to reduce the wavelength deviation. Detrimentally, this approach is rather complex and costly; it is limited by the precision and stability of the wavelength locker.
The prior art is lacking a simple and inexpensive reference laser source for emitting light at a precisely known wavelength.